1. Field of Invention
The present invention generally relates to ink jettable overprint compositions and their use in methods for protecting images on a substrate. The overprint compositions of the present invention provide a number of advantages to both ink-based and toner-based images, such as, for example, image permanence, thermal stability, lightfastness, and smear resistance.
2. Description of Related Art
In conventional xerography, electrostatic latent images are formed on a xerographic surface by uniformly charging a charge retentive surface, such as a photoreceptor. The charged area is then selectively dissipated in a pattern of activating radiation corresponding to the original image. The latent charge pattern remaining on the surface corresponds to the area not exposed by radiation. Next, the latent charge pattern is visualized by passing the photoreceptor past one or more developer housings comprising toner, which adheres to the charge pattern by electrostatic attraction. The developed image is then fixed to the imaging surface or is transferred to a receiving substrate, such as paper, to which it is fixed by a suitable fusing technique, resulting in a xerographic print or toner-based print.
Known methods of protecting toner-based images include adding wax to the toner and applying an overprint coating to the substrate. The overprint coating, often referred to as an overprint varnish or composition, is typically a liquid film coating that may be dried and/or cured. Curing may be accomplished through drying or heating or by applying ultraviolet light or low voltage electron beams to polymerize (crosslink) the components of the overcoat. However, known overprint coatings, such as those described in U.S. Pat. Nos. 4,070,262, 4,071,425, 4,072,592, 4,072,770, 4,133,909, 5,162,389, 5,800,884, 4,265,976, and 5,219,641, for example, fail to adequately protect toner-based prints and do not possess the requisite properties for controlled application, such as, for example, by an ink jet printer.
Typically, known coating formulations are applied using a liquid film coating device, and thus are often applied over the entire surface of the image, i.e., flood coating. Applying a composition to part of an image, i.e., spot coating, is possible, but it requires preparing a plate or cylinder prior to application of the overprint composition. Thus, applying known coating formulations can be inefficient, difficult, and time-consuming and is not desirable for integration with variable-data digital printing.
Coating formulations for ink-based images are also known. For example, UV curable ink-jet inks have been used in an attempt to overcome the failure of ink jet-generated images to withstand heat and sunlight. Typically, such UV curable ink-jet inks include polymerizable monomers, oligomers, or a mixture thereof along with a dye or pigment for color. However, these ink-jet inks often contain relatively large amounts of toxic solvent or water, as described in U.S. Pat. Nos. 4,978,969 and 5,623,001, respectively, or other toxic components, such as the varnish described in U.S. Pat. No. 5,270,368, or require specific, impractical conditions, such as, the varnish described in U.S. Pat. No. 4,303,924.
Accordingly, a need exists for an ink jettable protective composition that provides overprint coating properties including, but not limited to, thermal and light stability and smear resistance to toner-based images and ink-based images, particularly in commercial print applications. More specifically, a need exists for an overprint coating that has a low viscosity (to enable ink jetting), yet is stable at the generally high temperature required for ink jetting and achieves the desired properties, including, but not limited to, the ability to wet over silicone fuser oil (generally found on xerographic substrates), permit overwriting, reduce or prevent thermal cracking in response to thermal expansion of the toner, and protect an image from sun, heat, etc. The compositions and processes of the present invention, wherein a toner-based or ink-based image and/or substrate thereof is coated with an ink jettable, radiation curable overprint composition, satisfies this need.